This invention relates to an apparatus and method for measuring adsorption and desorption that precisely measures the volume of gas adsorbed on or desorbed from a solid sample. Specifically, this invention relates to an apparatus for measuring adsorption and desorption to calculate surface area, pore diameter distribution, pore volume and the like of the solid sample. This invention also relates to a method for measuring adsorption and desorption by using the apparatus.
For example, various particles such as solid catalysts, adsorbents, ion-exchangers, and ceramics are widely used as industrial materials. These particles vary in their physical properties such as specific surface area, pore volume and pore diameter distribution according to raw materials and manufacturing conditions. Consequently, the particles vary in industrial performance. When the particles are used as industrial materials, the specific surface area, the pore volume, and the pore diameter distribution of the particles should be precisely known.
Generally, a gaseous-phase adsorption method is used for measuring the physical properties of industrial-material particles. In this method, inert gas such as nitrogen gas is used as adsorbate gas, and a sample physically adsorbs the adsorbate gas. Variances in the pressure and volume of the adsorbate gas are obtained for measuring the adsorption of the sample. In this method, variances with time in adsorption pressure values and adsorbate gas volume at constant temperature are used. The configuration of adsorption isotherm, or hysteresis, seen on the adsorption isotherm between increase and decrease of the adsorption is, therefore, used.
In the related-art method, a batch-process apparatus is used. The batch-process apparatus comprises a measuring portion for measuring volume and pressure of gas and a sample container connected through a valve to the measuring portion. The measuring portion is provided with valves for supplying and exhausting gas to and from the measuring portion, respectively.
When adsorption is measured with the apparatus, a volume M of gas is first introduced into the measuring portion, and the measuring portion is connected to the sample container. When pressure in the measuring portion and the sample container is in equibrium and the adsorption of the sample is in equibrium, a series of processes for measuring pressure values is repeated. At each process, the volume M of gas is increased. The adsorption is calculated from the pressure values, the volume of the measuring portion, and the like. On the other hand, when desorption is measured, the measuring portion is evacuated, and the measuring portion is connected to the sample container. When pressure in the measuring portion and the sample container is in equibrium, a series of processes for measuring values is repeated. At each of the processes, desorbed gas is exhausted from the measuring portion. The desorption is calculated from the pressure values, the volume of the measuring portion, and the like.
However, since time is required until the pressure in the measuring portion and the sample container is in equibrium, the measurement of one sample takes eight to fifteen hours or longer. During the processes, an operator should timely and effectively introduce adsorbate gas into the measuring portion, exhaust desorbed gas from the measuring portion, and open or close the valves. The operator should be experienced in introducing the adsorbate gas and in adjusting the openings of the valves, but his error cannot be avoided to some degree.
A method for measuring adsorption and desorption using a mass flow controller is proposed in Japan Published Unexamined Patent Application No. 61-102538. This method is different from the above method using the batch-process apparatus in that gas is continuously fed little by little into a vacuum sample container with the mass flow controller. The flow rate of gas is substantially kept constant. When gas is continuously supplied to the sample container, adsorption and desorption of gas by a sample are calculated based on the flow rate of gas and measured variances in the pressure of the sample container. Adsorption isotherm is thus obtained. In the method, when gas is introduced into the sample container, the adsorption of the sample in the sample container is kept in equibrium. Pressure values are measured at several points of time from the beginning till the end of the introduction of gas into the sample container. Excessive time is not required for obtaining pressure values. An adsorption isotherm can be obtained in a relatively short period of time.
In the above disclosed method, the adsorption of the sample is calculated in a predetermined equation by using the flow rate of gas. The mass flow controller controls the flow rate of gas substantially constant at a target value. However, the mass flow controller should be frequently adjusted and calibrated so that the flow rate of gas should not deviate from the target value and the adsorption can precisely be calculated. Actually, the flow rate of gas cannot be kept constant, and it sometimes deviates from the target value. Consequently, precise calculation of adsorption or desorption cannot result.